* It's not about computers and them (Liz and Werner). It's about Cosmo.
* Computer dating is about algorithms and pattern matching. Cosmo didn't have any suspicion of Liz and Werner going on a date; even if he saw a type-mismatch, humans are complex and multi-faceted. But when he learned a computer program ostensibly made the match, his alarm bells went off.
Beyond that, there was already the suspected breach of the site which he knew Bish was interested in breaching. The mere mention of a computer being involved would have made him far more likely to suspect his involvement.
